Here on Earth the surfaces of pretty much all but the most freshly-exposed (geologically-speaking, of course) rock surfaces exhibit the effects of atmospheric weathering—from rain, snow, and ice to wind, dust-blown sand, flowing water, and extreme heat. And underlying all of that are the relentless forces of tectonic activity. But on dry, airless, and tectonically “dead” worlds in space like asteroids and many moons, rocky surfaces weather differently and much, much more slowly. The only rain comes in the form of occasional microscopic meteorite strikes and the only wind is charged atomic particles streaming out from the Sun into the vacuum of space. Both are incessant but their effects are, even on Earthly geologic time scales, extremely slow-acting.
Now, using the cameras aboard NASA’s OSIRIS-REx asteroid-sampling spacecraft, currently in orbit around the near-Earth asteroid Bennu, scientists have been able to observe the weathering effects of another factor ubiquitously found across the Solar System: the warming energy of sunlight.
Using the OSIRIS-REx Camera Suite (OCAMS), comprising three separate imaging instruments aboard the spacecraft, a team of scientists found evidence of thermal breakdown on some of the many boulders strewn across Bennu’s surface, in the form of “exfoliation” small-scale flaking and north-south directional stress fractures.
“This is the first time evidence for this process, called thermal fracturing, has been definitively observed on an object without an atmosphere,” said Jamie Molaro from Tucson’s Planetary Science Institute, lead author of a paper published on Nature Communications on June 9, 2020. “It is one piece of a puzzle that tells us what the surface used to be like, and what it will be like millions of years from now.”
“These findings provide substantive and compelling evidence that thermal fracturing plays an important role on airless body surfaces, which has major implications for understanding the evolution of asteroid surfaces, orbits, and populations.” (J. L. Molaro et al.)
Daily temperature fluctuations are extreme on asteroids, which can rotate fairly quickly; a full Bennusian day is only 4.3 hours long. Surface temperatures that face the Sun directly reach almost 127 degrees Celsius (260º F) during the day and sink to –73 degrees Celsius (–100º F) at night before starting all over again. (If Bennu’s day were longer, it would even get a bit hotter and considerably colder.) Over time this steady and extreme variation puts a lot of stress on the rocks.
But before OSIRIS-REx and OCAMS, we didn’t have good enough imaging technology so close to an asteroid (the image above was captured from less than half a mile away) to resolve the specific effects, like exfoliation.
And while other causes exist for exfoliation besides thermal variations, they simply aren’t present on an asteroid like Bennu.
The mission news release states “…rain and chemical activity can produce exfoliation, but Bennu has no atmosphere to produce rain. Rocks squeezed by tectonic activity can also exfoliate, but Bennu is too small for such activity. Meteoroid impacts do occur on Bennu and can certainly crack rocks, but they would not cause the even erosion of layers from boulder surfaces that were seen. Also, there’s no sign of impact craters where the exfoliation is occurring.” (Source)
This information should also help better determine the ages of surfaces on other airless worlds, which would be subject to the same thermal effects but in different ways depending on their orbits, rotational periods, compositions, etc. According to the OSIRIS-REx team: “In general, the more weathered a surface is, the older it is. For example, a region with a lot of craters is likely to be older than an area with few craters, assuming impacts happen at a relatively constant rate across an object. However, additional weathering from thermal fracturing could complicate an age estimate, because thermal fracturing is going to happen at a different rate on different bodies, depending on things like their distance from the Sun, the length of their day, and the composition, structure and strength of their rocks. On bodies where thermal fracturing is efficient, then it may cause crater walls to break down and erode faster. This would make the surface look older according the cratering record, when in fact it is actually younger. Or the opposite could occur. More research on thermal fracturing on different bodies is needed to start to get a handle on this, according to Molaro.”
Launched aboard a ULA Atlas V 411 on September 8, 2016, OSIRIS-REx will collect a sample of Bennu’s surface in August 2020 using its TAGSAM (Touch-and-Go-Sample-Acquisition-Mechanism) instrument and return it to Earth for study in September 2023.